Microsoft explains activation with Windows Vista

More clarification has come out from Microsoft regarding how Vista will treat changing hardware and activating the OS more than once. A lot of the fears have been misplaced, though overall Vista is still stricter than XP when it comes to keeping a system licensed properly. In particular, Vista will allow you to activate multiple times, just as Windows XP does. Changing most hardware, such as the CPU, RAM, video card and other components will not require re-activation, though swapping the motherboard and/or hard drive most likely will. If you do have to re-activate, assuming the amount of hardware changed is within the limit, you can do such up to 10 times. For most people, and in fact many enthusiasts, 10 times is probably enough to last the lifetime of a machine.

That won't help those who reinstall every 3 months or change motherboards all the time. In those cases, Microsoft would have to be contacted directly. At that point they can activate your machine again at their discretion, or choose not to. As expected, you can't have more than one machine activated at a time. Overall, it's not much different from XP. For or against activation, many companies aside from Microsoft use it and it is here to stay.